Generally, a wireless communication system including a base station (BS) and a user equipment (UE) provides various types of communication services such as voice, data and the like to user equipments through one or more base stations. In general, a single base station may cover at least one or more cells.
Recently, a structure of the wireless communication system has been changed from a type of a centralized base station based on a macro cell corresponding to the conventional vertical structure into a type based on various kinds of small cells such as a pico cell, a femto cell and the like interconnected with the macro cell.
In small cell enhancements for E-UTRA and E-UTRAN SI (study item) as one of 3rd generation partnership project (3GPP) standard scopes, ongoing research has been made in order to enhance indoor/outdoor scenarios using low power nodes.
Moreover, according to small cell enhancements for E-UTRA and E-UTRAN SI, a concept of dual connectivity, which enables a user equipment to have simultaneous connectivity to a macro cell layer and a small cell layer, has been applied.
In a heterogeneous network deployment in which the macro cell and various kinds of small cells such as the pico cell, the femto cell and the like for low-power/short-range communication coexist, random user equipments may be put in a dual connectivity state in a manner of being simultaneously connected to the small cells as well as the macro cell for throughput performance improvement and mobility enhancement.
If a dual connectivity user equipment moves, events such as change, removal and addition of the small cells may occur frequently, whereby transmission of radio resource control (re)-configuration messages may occur frequently.
In addition, if a random user equipment performs uplink synchronization (UL) in order to perform RRC (radio resource control) connection on a small cell, while the small cell belongs to sTAG (secondary timing advance group) different from sTAG to which an existing cell belongs (e.g., an inter band is used), the corresponding user equipment needs to perform an additional random access procedure.
Furthermore, in the case of the heterogeneous network deployment or in case of an environment in which only small cells exist without the macro cell, a random user equipment can be served by a specific small cell.
If the dual connectivity user equipment moves, a handover procedure event between small cells may be performed frequently. And, the random access procedure may be performed for the UL synchronization in the case of such a handover procedure. Moreover, if the handover procedure is performed, while a cell (or CC (component element)) served by a specific small cell belongs to sTAG (secondary timing advance group) different from sTAG to which the macro cell (or Pcell) belongs (e.g., inter band is used), an additional random access procedure is performed.
In the above-mentioned processes, each user equipment needs to independently perform a random access procedure in order to perform UL synchronization. Thus, the random access procedure may be performed frequently.
In this case, it may cause a problem of increasing burden on a base station and a user equipment that perform the random access procedure frequently and a problem that delay occurs frequently in the course of obtaining the UL synchronization. Therefore, the above-mentioned problems should be solved and methods of solving the problems are required.